Zener and Regular Diode for Flyback Handling

[Time]

Please refer to this post on another forum:

http://cr4.globalspec.com/comment/639843/Designing-Thyristor-Triggering-with-Pulse-Transformers

In his circuit he uses a zener diode in conjunction with a regular diode.  In my limited experience, this has struck me as peculiar.  I was hoping some people here could shed some light on why the zener diode?  I would think just the regular diode would be sufficient.

[SeanB]

The zener is there to allow a faster decay of the current during switch off. If you put a diode across the coil then turn on the switch current starts to flow and increases linearly with time. When the switch turns off the inductor tends to want to keep the current flowing, and if the diode is there it flows until it is dissipated in the diode drop and the coil resistance. With the zener diode the same power is now dissipated in a much higher resistance device than the coil so it decays faster. You can use a regular resistor and parallel capacitor there in place of a zener with a similar effect but with a trade off of the peak voltage across the switch being proportional to the pulse width, which means you need a switching device with a maximum voltage of at least 2x the supply rail and the diode has to have a pulse rating equal to the peak current in the coil. The zener gives a more closely defined peak voltage and a lower peak current through the diode. You also do not need to have as fast a switching diode there.

Another method is to use a zener diode across the switch, selected so as to not exceed the max rated off state voltage of the switch, though this device has to be a fast turn on transient suppression diode capable of handling the power in the coil.

[Time]

Awesome, thank you for your response.

What determines the zener voltage?

[SeanB]

Zener voltage lower limit is the supply rail, the upper limit is the Vceo of the transistor less the supply rail. Thus if you use a transistor capable of withstanding 600V and the supply is 300VDC from a mains rectifier you will select a diode of 300V breakdown. If you have a transistor with 1200V Vceo you can use any voltage from 300V to 900V, the higher the voltage the faster the turn off.

If you are using a 12V/24V rail then you would select the transistor to be a 300-400V part and use a diode of around 100V, or a transient suppressor or varistor of around that range.

[Paul Price]

The Zener diode is there to prevent the flyback pulse on the collector of the switching transistor from exceeding the maximum voltage rating of the transistors, but without it, the diode by itself would completely clamp the flyback pulse desired.. If the flyback pulse voltage is completely shunted back to the power supply +, then you have no flyback, no flyback output from the circuit and the circuit is useless if the flyback portion of the pulse is needed on the secondary of the transformer.

[Marco]

The flyback pulse isn't desired in the circuit though ... it's just unavoidable.

The zener is there to dissipate the magnetizing current (with only a diode a lot of it is going to heat up the transformer and if the pulses come to quick the current might even build up) and get the circuit ready for the next pulse, unlike a resistor it also limits the voltage on the MOSFET and the negative voltage on the Thyristor gate regardless of current.

[Time]

I appreciate everyone who has taken the time to respond to this.  You have all been a help.